ETUDE TRIDIMENSIONNELLE DE LA CAPACITE ULTIME DES PLAQUES D’ANCRAGE DANS UN SOL FROTTANT

Les plaques d’ancrage sont généralement de type tridimensionnel, le plus souvent carrées, circulaires ou rectangulaires. La présente étude s’intéresse au calcul numérique de la force limite d’arrachement d’une plaque d’ancrage horizontale et circulaire, en utilisant le code FLAC3D (Fast Lagrangian Analysis of Continua). La plaque d’ancrage est enterrée dans un sol frottant, régi par une loi élastique-parfaitement plastique et le critère de Mohr-Coulomb a été adopté. Le facteur d’arrachement a été calculé en considérant une plaque d’ancrage rigide, rugueuse et soumise à une force d’arrachement verticale et centrée. Les valeurs numériques obtenues ont été comparées aux formulations disponibles dans la littérature.  Plate anchors are generally square, circular or rectangular. This study focuses on a numerical study using the finite-difference code Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D), to evaluate the ultimate pullout capacity of horizontal circular anchors plate embedded in a frictional soil. The soil was considered to be a linearly elastic-perfectly plastic material, obeying Mohr–Coulomb criterion. The anchor break-out factors were calculated for a rigid and rough circular plate, subjected to centered vertical loads. The results have been compared with those available in the literatur

ETUDE NUMERIQUE DES PRESSIONS PASSIVES APPLIQUEES SUR UN BLOC D’ANCRAGE RIGIDE NUMERICAL STUDY OF PASSIVE EARTH PRESSURES AGAINST RIGID ANCHOR BLOCK

L’étude des pressions passives du sol joue un rôle très important, dans le cas des problèmes d’interaction sol-structure. Plusieurs paramètres géométriques et mécaniques influent sur l’intensité et la distribution des pressions passives; lorsque les blocs d’ancrage des ouvrages de soutènement sont indépendants et ne forment pas un corps continu, le sol n’est pas en équilibre à deux dimensions. Les blocs d’ancrage mobilisent la résistance passive du sol d’une manière tridimensionnelle. Cette étude vise, d’une part la présentation de quelques méthodes utilisées pour le dimensionnement des blocs d’ancrage, et l’estimation numérique des pressions passives sur un bloc rigide et isolé, soumis à un mouvement de translation dans un sol frottant; d’autre part l’évaluation de l’influence de la largeur d’un bloc sur les pressions passives, en utilisant le code FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions). Les valeurs numériques obtenues par cette étude sont comparées aux résultats issus par l’approche cinématique de l’analyse limite

APPLICATION OF 2D SURFACE ELECTRICAL RESISTIVITY TOMOGRAPHY TO DETECT THE UNDERGROUND CAVITIES A CASE SITE STUDY: TOLGA AREA (ALGERIA)

Electrical resistivity tomography method is considered among the best non-destructive and non invasive geophysical techniques for detecting and characterizing the underground cavities and associated disorders. The detection of cavities in urban areas is important to prevent against different causes of accidents related to possible collapse and subsidence. This study focused on the application of electrical resistivity measurements to locate and identify cavities in the soil. The survey site is located in Biskra City, south-east of Algeria. This field contains cavities of natural origin and different sizes. The procedure using the electrical resistivity tomography of Wenner array permitted to detect the cavities in the range of depth 2 to 4m. These cavities were caused by the dissolution phenomena of carbonate materials. Also mechanical and dynamic penetrometer tests were performed in order to propose a solution for foundations building

UNDRAINED BEARING CAPACITY OF EMBEDDED STRIP FOOTINGS UNDER VERTICAL AND HORIZONTAL LOADS

This paper is concerned with the undrained bearing capacity of embedded strip footing under inclined loading (i.e. combined vertical and horizontal). A series of numerical computations using the finite-difference code Fast Lagrangian Analysis of Continua (FLAC) was carried out to evaluate the failure envelopes in vertical force – horizontal force (V-H) plane, using both probe and swipe analyses. The adopted approach involves a numerical solution of the equations governing elasto-plastic soils. The soil is modeled by an elasto-plastic model with a Tresca criterion. The results are presented in terms of the failure envelope in vertical and horizontal loading plane. 

Dynamics of long bubbles propagating through cylindrical micro-pin fin arrays

The dynamics of two-phase flows confined within complex and non-straight geometries is of interest for a variety of applications such as micro-pin fin evaporators and flow in unsaturated porous media. Despite the propagation of bubbles in straight channels of circular and noncircular cross-sections has been studied extensively, very little is known about the fluid dynamics features of bubbles and liquid films deposited upon the inner walls of complex geometries. In this work, we investigate the dynamics of long gas bubbles and thin films as bubbles propagate through arrays of in-line cylindrical pins of circular shape in cross-flow, for a range of capillary and Reynolds numbers relevant to heat transfer applications and flow in porous media, different pitch of the cylinders and bubble lengths. Three-dimensional numerical simulations of the two-phase flow are performed using the open-source finite-volume library OpenFOAM v.1812, using a geometric Volume of Fluid (VOF) method to capture the interface dynamics. Systematic analyses are conducted for a range of capillary numbers Ca = 0. 04 --2R, Reynolds numbers Re = 1 -- 1000, streamwise pitch of the cylinders sx = 0. 125R, with R being the radius of the pin fins, and initial bubble length Lb = 2. 5R --12R. The simulations reveal that when bubbles propagate through pin fin arrays, they tend to partially coat the cylinders with a thin liquid film and to expand in the cross-stream direction within the gap left between adjacent cylinders. The liquid film deposited on the cylinders is significantly thinner than that reported for straight channels and similar geometrical constraints. As the streamwise distance between the cylinders is decreased, the flow configuration tends towards that for a straight channel, whereas larger distances cause the bubble to expand excessively in the cross-stream direction, and to eventually arrest when sx > 2R. Inertial effects have a strong impact on the bubble shape and dynamics when Re > 500, triggering time-dependent patterns that lead to bubble fragmentation and much thicker liquid films

NUMERICAL STUDY OF DEPTH FACTORS FOR UNDRAINED LIMIT LOAD OF STRIP FOOTINGS

Current studies of bearing capacity for shallow foundations tend to rely on the hypothesis of an isolated footing lying on the ground surface. In practice a footing never lies on the ground surface; it is mostly embedded at a depth D below the ground surface. This paper focuses on a numerical study using the finite-difference code Fast Lagrangian Analysis of Continua(FLAC),to evaluate the bearing capacity of embedded strip footings. The effect of the embedment is estimated though a depth factor, defined as a ratio of the bearing capacity of a strip footing at a depth Dto that of a strip footing at the ground surface. The results presented in this paper show that the size and shape of the shear zone and displacement field defining the undrained capacity of shallow foundations under centred vertical loading are dependent on embedment rati

NUMERICAL ANALYSIS OF SHALLOW FOUNDATIONS ON PURELY COHESIVE SOIL UNDER ECCENTRIC LOADING

The classical theory of the bearing capacity of shallow foundations is based on the effective width approach for the case of vertical eccentric loading. This method stipulates that the area of the foundation, used in the calculation of the bearing capacity, is equal to the area of a fictive foundation on which the loading is applied at the center. This paper evaluates the performance of this approach in predicting the ultimate load of the footing. A numerical analysis is performed to estimate the undrained bearing capacity mobilized in a purely cohesive soil under a strip footing, using the finite difference code FLAC3D (Fast Lagrangian Analysis of Continua in 3 dimensions). The results of this analysis show that the effective width approach provides a good approximation of the bearing capacity for this kind of problems

Conjugate heat transfer effects on flow boiling in microchannels

This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of Dh=229 µm, a uniform base heat flux of qb=100 kW/m2, and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range ϵ=0.25–4 and Wf=Dh/8−Dh, respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from Nu≃4 and Tb−Tsat≃9K when ϵ=4, to Nu≃6 and Tb−Tsat≃2K when ϵ=0.25, for same fin thickness Wf=Dh/8. In the two-phase flow regime, Nusselt numbers in the range Nu=12−36 are achieved. The trends of the Nusselt number versus the aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, Wf=Dh/8 and Wf=Dh/4, an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range ϵ=0.5–2 the Nusselt number appears weakly dependent on ϵ. For thicker fins, Wf=Dh/2 and Wf=Dh, the Nusselt number decreases slightly when increasing the aspect-ratio in the range ϵ=0.5–2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with ϵ<1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits

Numerical investigation of bubble dynamics and flow boiling heat transfer in cylindrical micro-pin-fin heat exchangers

Micro-pin-fin evaporators are a promising alternative to multi-microchannel heat sinks for two-phase cooling of high power-density devices. Within pin-fin evaporators, the refrigerant flows through arrays of obstacles in cross-flow and is not restricted by the walls of a channel. The dynamics of bubbles generated upon flow boiling and the associated heat transfer mechanisms are expected to be substantially different from those pertinent to microchannels; however, the fundamental aspects of two-phase flows evolving through micropin-fin arrays are still little understood. This article presents a systematic analysis of flow boiling within a micro-pin-fin evaporator, encompassing bubble, thin-film dynamics and heat transfer. The flow is studied by means of numerical simulations, performed using a customised boiling solver in OpenFOAM v2106, which adopts the built-in geometric Volume of Fluid method to capture the liquid–vapour interface dynamics. The numerical model of the evaporator includes in-line arrays of pin-fins of diameter of 50 μm and height of 100 μm, streamwise pitch of 91.7 μm and cross-stream pitch of 150 μm. The fluid utilised is refrigerant R236fa at a saturation temperature of 30 ◦C. The range of operating conditions simulated includes values of mass flux = 500–2000 kg∕(m2 s), heat flux = 200 kW∕m2 , and inlet subcooling = 0–5 K. This study shows that bubbles nucleated in a pin-fin evaporator tend to travel along the channels formed in between the pin-fin lines. Bubbles grow due to liquid evaporation and elongate in the direction of the flow, leaving thin liquid films that partially cover the pin-fins surface. The main contributions to heat transfer arise from the evaporation of this thin liquid film and from a cross-stream convective motion induced by the bubbles in the gap between the cylinders, which displace the hot fluid otherwise stagnant in the cylinders wakes. When the mass flow rate is increased, bubbles depart earlier from the nucleation sites and grow more slowly, which results in a reduction of the two-phase heat transfer. Higher inlet subcooling yields lower two-phase heat transfer coefficients because condensation becomes important when bubbles depart from the hot pin-fin surfaces and reach highly subcooled regions, thus reducing the two-phase heat transfer